This invention relates to analog signal processing, and more particularly to radio transceivers having the capability of sensing clear channels.
This invention further relates to the transceiver and components thereof described and claimed in the following U.S. Patent Applications filed of even date with and assigned to the assignee of the present invention: U.S. Ser. No. 791,611 entitled "A Digitally Transmitting Transceiver" by Edward R. Caudel and William R. Wilson; U.S. Ser. No. 791,629 entitled "A Clarifying Radio Receiver" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,449 entitled "An Automatically Clarifying Radio Receiver" by Michael J. Cochran and Edward R Caudel; U.S. Ser. No. 791,254 entitled "A Computer Controlled Radio System" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,450 entitled "A Transceiver With Only One Reference Frequency" by Michael J. Cochran; U.S. Ser. No. 791,253 entitled "A Transceiver Capable of Sensing A Clear Channel" by Jerry D. Merryman, Et.Al.; U.S. Ser. No. 791,265 entitled "A Signal Strength Measuring Transceiver" by Edward R. Caudel; U.S. Ser. No. 791,256 entitled "A Highly Selective Programmable Filter Module" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,616 entitled "A Dual Processor Transceiver" by Edward R. Caudel, William R. Wilson and Thomas E. Merrow; U.S. Ser. No. 791,264 entitled "An Electronic Phase Detector Circuit" by Michael J. Cochran. Radio systems receive radiated electronic input signals comprised of a plurality of non-overlapping frequency bands, filter one of the bands from the plurality, frequency shift the one band from radio frequencies to a lower frequency, and convert the selected band to audible sounds. The input signals may be amplitude modulated (AM), or single sideband (SSB) signals as an example. Antenna means receive the radiated input signals. Filtering devices are included in the radio system to select one of the bands from the plurality. Mixing devices are included to frequency shift the selected band. And demodulator devices are included to demodulate the selected band.
The radio system herein described includes a charge transfer device transversal filter as a main component. The charge transfer device both filters and mixes signals applied to its input. In the past, filtering and mixing operations were performed by separate circuits. Thus, one advantage of the present invention is that it eliminates some mixing circuitry that the prior art required.
Another advantage of the present invention is that it includes a single charge transfer device filter for selectively filtering both single sideband input signals and AM input signals. This filtering is performed by a charge transfer device filter having passbands whose bandwidth and location in the frequency domain are dependant upon the frequency of the filters clocking signal. When the charge transfer device filter is clocked with one frequency, it passes AM signals, and when it is clocked with another frequency, the filter passes single sideband signals. In the past, separate filters were required for AM filtering and for single sideband filtering.
Also, in the past, the filtering operation was performed by electronic circuits which were both expensive and too large to integrate on a semiconductor chip. This was because the narrow spacing between adjacent frequency bands of the input signal required the filter to have an abrupt transistion from its passband to its stopband. The width of this transistion is generally referred to as the skirt response of the filter. In order to achieve the required narrow skirt response, either multipole mechanical filters, lumped inductor-capacitor elements, or multipole crystal filters were required. In comparison, the charge transfer device incorporated into the present invention has the necessary sharp skirt response, and in addition is both inexpensive and capable of being integrated on a single semiconductor chip.
Accordingly, it is one object of the invention to provide an improved radio system.
It is another object of the invention to provide a radio system having a filter with passbands of a selectable width.
Another object of the invention is to provide a radio system having a device which both filters and mixes its input signal.
Still another object of the invention is to provide a radio system having a filter having a skirt response sharp enough to filter one frequency band from a plurality and also capable of being integrated on a single semiconductor chip.